A trailer, of any size and design, is generally required to be provided with such feature as one or more tail lights and brake signals, connected to and synchronized with the corresponding elements of the towing vehicle. Since trailers must be detachable, permanent wiring between the trailer and the towing vehicle cannot be installed and it is common practice to terminate the upstream ends of the trailer light wires in a 3- or 4-wire plug or socket element, for separable connection to a complementary element wired to the electrical system of the towing vehicle. The wires on both sides of such a connection are necessarily unsupported for substantial distances unless wrapped around parts of the trailer or towing vehicle, so that the plug and socket may dangle below the hitch in a vulnerable position.
When a trailer is not connected to a towing vehicle, its light wires and their plug or socket need restraint and support to keep them clean and safe, ready for use when the trailer is to be towed. An expedient commonly resorted to is the securement of loose wires to the trailer tongue by means of an electrical tape or the like.
A more cumbersome and expensive expedient involves the provision of a socket fitted in a hole drilled in the trailer tongue, the wires and the trailer circuits being connected to terminals in the socket, the corresponding wires from the towing vehicle terminating in a complementary plug which is engaged with the trailer tongue socket when the trailer is being towed.